Electronic mail, or email, is the exchange of computer-stored messages over electronic networks. Users of email systems typically have the ability to transmit email messages to each other utilizing these electronic networks. Coordinating the transmission of email is typically handled utilizing an email server, or mail transfer agent (MTA). The MTA coordinates the receipt and delivery of email messages.
A recurring problem for current email systems is that unsolicited email has become a nuisance for users. These unsolicited emails are commonly referred to as “spam”. Indeed, due to the low cost of sending emails and due to the ease with which email addresses may be harvested, some individuals or businesses send spam to a large number of individuals. Spam not only occupies much of the storage space, computing resources, and traffic bandwidth of email systems, but it also wastes the recipient's time in downloading, reading, and deleting the spam. In addition, spam can also be used for phishing scams or for virus attacks. These phishing scams may involve leading unsuspecting users to counterfeit websites to obtain financial information such as usernames, passwords, and account numbers. Likewise, virus attacks may include virus, Trojan, malware, and other programs designed to harm the user's computer.
Similarly, telephone or mobile phone communications have accounts with unique phone numbers as the addresses identifying authorized subscribers. A call message within a telephone or mobile phone communications system is routed between two local exchanges for two communicating parties. However, telephone spam and mobile phone spam is becoming more popular as their cost of usage gets lower. These kinds of spam are junk text messages or voice messages delivered to the receiving device of the victims. Phone spam may interrupt the normal life of the receiver, and may need more time to be cleared, especially for voice messages. Likewise, cellular phone users may be charged for each digital message received, they may find themselves paying a premium for the privilege of receiving unsolicited advertising. Moreover, the newly emerged session initiation protocol (SIP), used for Internet telephone calls, multimedia distribution and message notification between two user agents, will face much worse spam problems because of its low cost and popularity across various networks.